The conclusion is merely that laissez-faire economics has some inaccuracy, not that it’s entirely inaccurate. So there’s no need to assume that anything makes laissez-faire economics entirely inaccurate.

The author only considers min-wage job availability in the fast-food sector, but he reaches a conclusion about total min-wage job availability everywhere. So he must assume that other sectors behave similarly to fast food. Otherwise, if the fast-food sector weren’t representative, then it couldn’t support any broader conclusions about the overall min-wage job market.

Too strong. The author argues that the net number of min-wage jobs doesn’t decrease. This allows for the possibility that the number of min-wage jobs decreases in some businesses and increases in others. So there’s no need to assume that no business has ever been found to decrease min-wage jobs.

The argument is purely about whether, and how, the minimum wage affects the number of min-wage jobs. What goes on with the average wage is irrelevant.

The argument is purely about changes in the number of min-wage jobs. The author doesn’t need to make any assumptions about the net availability of all jobs, or about how the unemployment rate reflects that availability. It’s possible that when the minimum wage increases, min-wage jobs stay put but other jobs are lost. That wouldn’t affect the argument.